This invention relates to tower packings that are used for mass transfer applications in chemical processes. Mass transfer is generally defined as the transfer of a component or components between two mutually immiscible phases. The component transferred may be a physical ingredient of a mixture such as a volatilizable liquid, in which event a property such as heat may simultaneously be transferred.
Mass transfer is generally achieved by contact between two immiscible fluids and most commonly between a gas and a liquid. For the mass transfer to be most efficient, the components must have a very large area of contact and for this reason many devices, called "packing materials", have been created to maximize the contact area within chemical process towers wherein mass transfer takes place. There are two basic varieties of "packing materials" that are used in such towers: structured and dumped. Structured packings are designed to be fixed in place and located such that liquids spread over the surfaces in a thin film so as to maximize the area available for contact with a gas or immiscible fluid contacting the liquid so spread. The shapes are commonly in the form of corrugated sheets laminated together with the lines of the corrugations alternating right and left on either side of an axis of symmetry. Dumped packings are shapes that can be dumped into a tower to provide a large surface area without compacting together to produce an excessively high pressure drop across the tower or localized blockages. The shapes of these dumped packings are many and varied including saddles, rings, arches with bridge loops, "snowflake" shapes and the like.
It is clearly important for effective use of such packings that the process liquid wets the surface rather than forming rivulets or beads on the surface. There are several ways of expressing the wettability of a packing surface by a liquid. One measure is the critical surface tension of the solid material. Any liquid with a surface tension less than the critical surface tension of the solid will completely wet the surface. On the other hand it will not completely wet the surface if the liquid surface tension is greater than the critical surface tension of the surface material. Thus the material from which the packing is made will often be selected with this criterion in mind.
For many applications in which one or more of the components is corrosive or chemically active against otherwise suitable materials, expensive or compromise materials may have to be selected. Often however this is only a partial solution because the surface tension of the liquid varies from the top of the tower to the bottom as the composition changes as a result of mass transfer.
These problems are particularly acute in the distillation of dilute acids to remove water and concentrate the acid. The surface tension of pure water is very high, (about 72 dynes/cm), and does not wet the materials that might typically be used such as stainless steel which has a critical surface tension of about 46 dynes/cm. Thus while the more concentrated solution at the foot of the tower may wet the surface of the packing, the surface initially contacting the more dilute solution whose surface tension approaches that of water may not be fully wetted resulting in relatively poor mass transfer.
One measure of efficiency of mass transfer is the HETP value which is the height equivalent of a theoretical plate, (the "stage"), that is the height of the theoretical stage required to achieve a fixed amount of mass transfer. Clearly the lower the HETP value, the more efficient is the mass transfer taking place.
The present invention provides a method of achieving improved wettability of the surfaces of metal tower packings and consequently improved efficiency of mass transfer. The invention further provides metal tower packings with improved wettability and enhanced critical surface tension. In addition the invention provides metal tower packings particularly suited for the concentration by distillation of dilute acids.